In central coolant systems for machining operations, soluble oil coolant is circulated to individual work stations, e.g., machining or parts washing equipment, to cool the parts being processed and to flush machining waste from the parts. The dirty coolant flows to a common filtration station, where the liquid coolant is separated from the solid waste, and the coolant is then recirculated back to the work stations.
A conventional, high volume, fixed media or drum filtration system, such as that disclosed in U.S. Pat. No. Re. 32,135, will remove solid waste to obtain a coolant clarity level of about 10-20 PPM (parts per million) particles of a size of about 10 to 15 microns.
Finer degrees of filtration are required for operations where the clarified coolant must flow through extremely small ports or apertures. For example, in a gun drill and in certain washer applications, clarification to a level of 3-5 PPM and a size of 3 to 5 microns is required. Further, washer fluids for washing machining waste from an engine block, for example, may contain detergents which leave a residue on the washed parts to which even the smallest particles may adhere. Also, the coolant may be rendered unusable after long-time operation by the accummulation therein of "fines" or particles passing through the filtration system and remaining in the circulation flow. For these applications requiring a finer degree of particle removal, filtration through paper media is required. To obtain the requisite particle removal, a "tight" paper is necessary, but the flow rate must be greatly reduced.
The present invention proposes a novel filtration system utilizing both a fixed media drum filter and a media paper filter disposed in a single tank to provide an input flow of filtrate at two distinct clarity levels. The drum filter delivers a high volume flow of filtrate at a first clarity level, e.g., 10 to 20 parts per million of from 10 to 15 micron particles; and the paper filter delivers a substantially smaller volume flow per square foot of surface area at a second clarity level, e.g., 3-5 parts per million of 3-5 micron particles.
The two distinct types of filters are immersed in a common tank full of dirty liquid. The drum filter removes the larger particles from a high volume of liquid, the particles accreting onto the drum surface for periodic removal, as by a fixed scraper against which the drum rotates. The particles removed from the drum usually agglomerate and sink into the common tank. The paper filter is located at the bottom of the common tank to filter liquid from the tank at a much lower flow rate, but at a higher clarity level. The paper is indexed according to the pressure drop across the paper as the paper clogs from the dirt accreted thereon.
Liquid is pumped by a clean liquid pump from the interior of the drum, and the same or a different clean liquid pump pumps clean liquid from the vacuum chamber underlying the paper media.
The contaminants scraped from the drum are removed from the tank by a scraper conveyor which also serves to move the paper as it is indexed over the vacuum chamber.
Typically, the paper of the paper filter is a very "tight" paper to give better cleanliness of the filtered liquid, and the flow rate through the paper is quite low, e.g., on the order of 5 gallons per minute per square foot. Under these circumstances, a clarity of 3 to 5 PPM and an average particle size of 3 to 5 microns can be maintained.
Further, because of the vacuum chamber at the bottom of the dirty liquid tank, the accreted dirt scraped from the drums is pulled downwardly toward the paper filter, either to be filtered onto the paper or to be carried out by the scraper conveyor. Consequently, a smaller than usual dirty liquid tank with minimal retention time, on the order of 1 to 2 minutes can be provided, rather than a much larger tank normally required for the 5 to 7 minutes retention time necessary for settling of the dirt scraped from the drum filter.
The combined filtration unit of the present invention can be used in either one of two modes of operation. Where the liquid of greater clarity is required for a separate operation, such as gun drill or the like, the paper filter vacuum chamber may be evacuated by a separate pump which is utilized to convey on the paper-filtered liquid to the gun drill working stations, while the liquid of less clarity from the drum filters is supplied to other machining operations. Where the paper filter is utilized in a "polishing" mode to remove fines from the coolant on a bypass-type operation, a single pump is utilized for both filtration units with about about 20% or less, preferably 10% or less of the liquid being filtered at the paper filter. Over a period of time, it has been found that such a bypass operation effectively removes the fines from the circulating coolant liquid and materially prolongs the life of the coolant.